Fix an off by one error when remapping MSI-X vectors.

pci_remap_msix() can be used to alter the mapping of allocated
MSI-X vectors to the MSI-X table. The code had an off by one error
when adding the IRQ resources after performing a remap. This was
fatal for any vectors in the table that used the "last" valid IRQ as
those vectors were assigned a garbage IRQ value.

MFC after: 3 days